This application is related to co-pending patent application Ser. No. 09/327,717, filed Jun. 8, 1999, titled xe2x80x9cExtended Wear Canal Hearing Device,xe2x80x9d referred to herein as xe2x80x9cthe ""717 application,xe2x80x9d assigned to the same assignee.
1. Technical Field
The present invention relates generally to miniature batteries, and more particularly to miniature batteries that are asymmetrical to fit in irregularly shaped cavities such as ear canals for hearing aid applications.
2. Description of the Prior Art
(1) Conventional Hearing Aid Batteries
Hearing aids and other miniature devices primarily rely on button cells which are symmetrically round as shown in FIG. 1. The cell is generally constructed of anode and cathode cans, which provide structural support as well as electrical conductivity to the device connected thereto. The metal encapsulation also provides sealing for the active electrochemical material within. Metal cans employed in a typical button cell battery are generally formed from a steel strip having nickel coating or nickel-copper combination depending on the application. The can generally has a thickness range of from 0.1 to 0.2 mm. A button cell battery for hearing aid applications is highly symmetrical having a circular cross section with diameter (D) as shown in FIG. 2. Zinc air cells, which rely on air (oxygen) in the environment as a source for their internal electrochemical reaction, are widely used in hearing aid applications due to their exceptional volumetric energy efficiency.
(2) Brief Description of the Ear Canal and Canal Hearing Aids
The human external ear 10 is comprised of the pinna 12 and the ear canal 11, which is narrow and tortuously elongated as shown in FIG. 3. The ear canal 11 begins laterally with the canal aperture 13 and terminates medially with the eardrum 16. First and second bends, 14 and 15 respectively, are generally present and must be considered when designing a deeply inserted canal hearing device. The ear canal generally has an oval cross section (FIG. 4) with a long diameter (CDL) and short diameter (CDS).
Recent advances in miniaturization of electronics, transducers and batteries have led to the development of canal hearing aids which are partially or deeply inserted in the ear canal. Canal hearing devices are generally inconspicuous, thus very desirable for the hearing impaired who may be concerned with the social stigma and vanity associated with wearing a visible hearing aid. However, due to the complex shape of the ear canal, being elongated, contoured and oval at the cross section, the conventional cylindrical button cells used in standard canal hearing devices are inefficient in terms of volumetric utilization of the ear canal cavity. For this reason among others, hearing aids are notorious for requiring frequent battery replacement and being uncomfortable, particularly for small ear canals.
(3) Review of State-of-the-Art in Hearing Aid Related Battery Technology Cretzmeyer, et. al. (U.S. Pat. No. 4,189,526), Smilanich U.S. Pat. No. 4,404,266) and Oltman et al. U.S. Pat. No. 5,582,930) describe zinc-air button cell batteries with various improved features. These button cell batteries are made of a cathode (positive) metal can crimped over an anode (negative) metal can with a sealing element therebetween (i.e., 2 in FIG. 2 of the ""930 patent). The two cans structurally support the battery assembly while providing electrical conductivity to a hearing device on the bottom and top as shown in FIG. 2. of the ""930 patent. Although each metal can may be made relatively thin, in the range of 0.114 to 0.15 mm, the presence of two parallel walls on the circumference of the cell (2 and 3 in FIG. 1), in addition to a seal (7 in FIG. 1) therebetween, presents a triple-wall effect which adversely affects the volumetric efficiency (battery longevity/volume) for the battery assembly. This results in shorter battery life and frequent battery replacement. A battery compartment with electrical contacts also consumes space further reducing the volumetric efficiency of the hearing device. Another source of volumetric inefficiency in conventional hearing aids is the acrylic outer shell which represents a secondary enclosure with respect to the battery assembly within, thus reducing the volumetric efficiency of the canal hearing device.
Oltman et al., in U.S. Pat. No. 4,687,714 discloses a battery assembly including a plastic outer shell which is deformable to accommodate volumetric expansion of the anodic material during its oxidation. The cathode terminal is formed by a metallic plate (cover) which is crimped over a flanged portion of the plastic shell as shown in FIG. 1 of the ""714 patent. The application of this battery for canal hearing aids is not practical for several reasons including: (1) the deformation and expansion of the shell may cause the battery to leak its harmful contents in the ear canal, particularly at the shell-cover joint; (2) the form factor of the battery assembly as described will not fit optimally in the ear canal, as the battery is button cell-like with a circular diameter and short height. Therefore, this plastic battery is more suited for non-medical applications such as industrial lighting as stated in the ""714 patent (line 20, col 2).
Aceti et al., in U.S. Pat. No. 6,058,198 describes a hearing aid assembly with a battery integral therewith. According to the ""198 patent, the anodic material and air-cathode assembly are contained within a unitary plastic housing along with electronic circuit and transducers (speaker and microphone). This approach eliminates the concept of a battery subassembly, thus appearing more efficient in its volumetric energy efficiency. However, combining anodic elements along with hearing aid components is not practical due to the corrosive effects of battery chemicals on circuits and transducer components within the hearing aid. Furthermore, eliminating a battery subassembly complicates the manufacturing process since the battery function cannot be independently tested or verified prior to its incorporation into the hearing device.
Leedom in U.S. Pat. No. 5,825,896 describes a hearing aid having two hinged boot portions, one of which incorporates a battery and a speaker. The ""896 patent describes the shape of the boot portion as having an elliptical cross-section which is advantageous for fitting deeply and more comfortably in the ear canal. The patent does not describe the nature of the battery assembly or its casing, but presumably it is metallic since the outer surface of the battery is shown connected to a metal leaf spring (110 in FIG. 6 of the ""896 patent) for providing electrical conductivity as disclosed.
In the aforementioned related ""717 application, Shennib et al. describe a battery assembly having a single-walled shell with an oval cross section when combined with a section of a hearing aid connected thereto. The battery assembly in the ""717 application is elongated and oval in cross section thus suited for fitting deeply and comfortably within the ear canal. Similarly, the ""717 application does not describe the nature of the battery assembly or its casing, but presumably it is metallic.
A key goal of the present invention is to provide a leakage-resistance battery assembly having irregular shape with non-circular cross section thus fitting optimally into anatomical cavities, particularly an ear canal.
Another goal of the invention is to provide a battery assembly having a thin plastic shell structure suited for easy and inexpensive manufacture in a variety of complex shapes for maximizing volumetric energy efficiency when incorporated into a canal hearing device.
Another goal of the present invention is to provide a disposable hearing aid, incorporating a plastic battery assembly, and specifically suited for extended wear applications in which a hearing aid is worn continuously in the ear canal for a relatively long period of time, preferably several months.
The invention provides a battery assembly constructed of an elongated plastic shell irregularly shaped to fit optimally in anatomical cavities, particularly ear canals. The battery assembly of the invention relies structurally on a thin plastic shell which can be inexpensively formed in a variety of complex shapes, thus offering superior volumetric utilization and energy capacity when compared with conventional cells employing metal button cell designs.
In a presently preferred embodiment, the battery assembly comprises a base unit, which caps the opening of the elongated plastic shell and seals the contents of the battery assembly within. During manufacture, the elongated plastic shell and the base unit are joined together in a sealing manner by welding or other suitable bonding processes. Once the battery assembly is manufactured as a subassembly, it is then incorporated into a miniature canal hearing device for powering the electrical and transducer components within.
In the preferred embodiment, the battery assembly is configured elongated along the longitudinal axis of the ear canal when placed within, and oval (i.e. elliptical) in its cross section. The medially tapered, or domed, shell relies on its unique configuration to distribute axial stresses, and thus can be made thin with sufficient strength although made of plastic. The tapering also allows an associated hearing device to fit optimally and comfortably deep in the ear canal. The battery assembly""s non-conductive biocompatible plastic structure adapted for essentially direct exposure to the environment of the ear canal, thus eliminating the need for a secondary enclosure which is required for conventional button cells. A hearing device employing the battery assembly of the invention can be made smaller and volumetrically more energy efficient, thus more suitable for extended wear applications.
In the preferred embodiment of the battery assembly, the hearing aid in which it is incorporated is disposable, intended to be discarded when the electrical energy of the battery assembly is depleted.